In a typical electric water heater, or in other types of electric liquid heating apparatus, the liquid to be heated is disposed in a vessel into which a resistance-type submersible electric heating element projects and may be selectively energized to heat the liquid to a predetermined temperature. As commonly manufactured, the vessel is of a ferrous metal material, such as steel, which is lined with a protective ceramic or other material to inhibit corrosion of the metal which can lead to leakage of the vessel. Corrosion is essentially an electrochemical phenomenon, and cathodic protection is a commonly used method of combating it.
The most common techniques for providing such cathodic protection for the liquid-containing vessels or tanks of electric liquid heating apparatus are (1) utilizing an erodable sacrificial anode in the vessel, and (2) using an impressed current cathodic protection (ICCP) anode in the vessel. The sacrificial anode technique has the advantages of low cost, continuous protection without any limitations such as power outage, software malfunction, etc. However, it has a limited life, and must be inspected periodically to verify that it has not dissolved to an extent rendering it incapable of carrying out its cathodic protection function. The ICCP anode protection technique provides several advantages over the sacrificial anode approach to cathodic corrosion protection, such as being capable of being precisely controllable with respect to its potential and current outputs, and having potentially unlimited operational life. However, the ICCP anode protection technique has some disadvantages such as requiring an associated control system and being more complex and costly than a sacrificial anode system.
As can be seen from the foregoing, it would be desirable to provide an electric liquid heating apparatus having incorporated therein improved cathodic corrosion protection apparatus, based on either a sacrificial anode approach or an impressed corrosion inhibiting voltage approach. It is to this goal that the present invention is primarily directed.